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Thread: BD flow questions

  1. #1
    Tategoi Loco4Koi's Avatar
    Join Date
    Jul 2012

    BD flow questions

    I have a 4" aeraed BD in 6' of water in a 6000 gallon pond. The line runs horizontally for 20' makes a long sweep 90° turn then about a 30° for 20 ft to my filter pit... The outlet is about three feet below pond water level....

    What should my expected flow be?
    Is there anyway to increase the flow.... I was thinking if I add an airline in the entrance of the drain, the air might push the water causing it to suck in more... Will this or anything else increase the flow?

  2. #2
    MCA is offline
    Honmei MCA's Avatar
    Join Date
    Dec 2003
    The main driver is gravity....hence gravity fed filter system. When a pump removes water in the filter pit, the column or water and air in the chamber weighs less that than the column of all water in the pond. So gravity moves the water to equal out the weight difference. Since the pump is constantly removing water....gravity is trying to keep up and creates the from from the BD to the filters.

    The flow rate is impacted by the height difference between the water in the pond and the level in the filter chamber. Greater height difference equals greater weight difference equals greater flow due to more water pressure.

    The main flow limiter is pipe and fittings resistance. For 4" pipe most folks will get 3000-4000 gph with gravity flow. If you want more you have to either have fewer fittings, less pipe, larger pipe, greater height difference (more pull from the pump),......or give up on gravity feed and do flooded suction where the BD pipe feeds pumps directly. I would try to avoid flooded suction as the pump will break the mulm into smaller pieces.

    I have already decided that if/when I rebuild our pond, the BD lines will be 6", not 4". You could connect a 6" BD line to both of the 4" ports on an Ultrascieve and get something like 8000gph flow from each BD. That would be a great flow rate for feed bakki showers.
    Koi keeping is not a belief system; it is applied science with a touch of artistry.

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